Optical position gauge

ABSTRACT

Apparatus for measuring the direction, extent and rate of linear  displacet of a body (e.g. a steel bulkhead) subjected to a harsh environment such as shock from an explosive force. A housing containing an extendable piston is connected between the movable body and a fixed structure. The piston has a surface marked with alternating high and low light reflecting bands which pass under a pair of light emitting optical fibers fixed on the housing. Light reflected from the bands is detected and converted into corresponding out of phase electrical signals for measuring direction, extent and rate of linear displacement of the piston.

BACKGROUND OF THE INVENTION

This invention is concerned with an instrument for measuringdisplacement of a body or object in a harsh environment such as ispresent in the vicinity of an underwater explosion. Instruments formeasuring displacements due to explosive forces are, in the processoften destroyed or rendered ineffective themselves.

There are three basic methods of measuring displacement: (1) mechanical,(e.g., deformable material, such as lead cones); (2) acoustic timing(e.g., transmit and receive); and, (3) accelerometers. Lead cones havelimitations in their use in measuring single axis deflections. The leadcone is placed between a body or object to be moved and a rigid (fixed)structure. The cone collapses upon movement of the body or object towardthe rigid structure. The disadvantage of the lead cone is that itmeasures only maximum displacement in one direction and cannot bere-used, and is subject to shock damage. Acoustic devices are basicallypressure measuring devices. Shock waves may "blind" receivers duringcritical measuring times. Thus, acoustic devices are not ideal inexplosive environments. Accelerometers deliver a signal proportional tothe acceleration of the moving body (object). To determine single axis(linear) displacement requires double integration of the data withrespect to time. During this double integration, errors tend to add andnot average out. Small accelerations and timing errors lead toinaccurate displacement measurements.

In the setting where the present invention will be used, it will bedesired to measure the direction, extent and rate of linear displacementof a body relative to a rigid (fixed) structure, for example. It isdesirable to obtain these data in an explosive environment withoutexperiencing destruction or malfunction of test equipment.

SUMMARY OF THE INVENTION

There is disclosed herein an instrument for measuring movement of a bodyrelative to a fixed position. It includes a housing containing anextendable and retractable piston which extends from the housing. Thehousing is adapted to be connected to a fixed structure, and the outerend of the piston is adapted to be connected to the body which is to besubjected to movement by an explosive force. Upon movement of the bodydue to the explosion, the piston is caused to move inside the housing.The piston includes a surface provided with side by side bands ofalternating high and low light reflectivity. Light sources positionedinside the housing and terminating in a pair of linearly spaced apartoptical fiber ends which illuminates two small linearly spaced apartspots on the piston surface. As the piston moves in the housing, itsbands pass underneath the light spots and relative reflectivity isdetected and converted into electrical signals having similarcharacteristics. The signal is read out to provide information as todirection, extent and rate of linear piston displacement, thus,providing the desired information regarding movement of the body whichhas been subjected to the explosive shock.

It is, therefore, an object of the invention to provide an arrangementembodied in an instrument for measuring direction, extent, and rate oflinear displacement of an object subjected to a shock such as from anexplosion in water.

It is another object of the invention to provide an instrumentconnectable between a fixed structure and a body to be subjected toexplosive shock including means generating optical signals convertableto electrical signals having similar characteristics for indicatingdisplacement characteristics on the body.

Still other objects of the invention will become apparent to one uponreading the specifiction in conjunction with drawings which form a partthereof.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a cross-sectional view of the apparatus according to theinvention illustrated in association with a body subjected to anexplosive force.

FIG. 2 is a detailed illustration of lands and grooves to definereflective bands on the piston.

FIG. 3 is an illustration of optical fibers in light couplingrelationship.

FIG. 4 illustrates a principle of the invention.

FIG. 5 is similar to the illustration in FIG. 4 but includes anadditional optical arrangement.

FIG. 6 is a block diagram illustration of electronic processing ofreflected light.

FIG. 7 is a schematic illustration of the electrical processing ofreflected light.

FIG. 8 illustrates piston displacement.

FIG. 9 illustrates recorded read out of piston movement in both lineardirections.

DETAILED DESCRIPTION OF THE INVENTION

For a detailed understanding of the invention, refer first to FIG. 1where there is shown in cross section a device, identified generally bythe numeral 10, comprising housing 12 provided with a cylindricalopening 14 in which piston 16 is positioned for linear displacement.Housing 12 is also provided with a cavity 18 including an extendedportion 20 having small openings for allowing optical fibercommunication with cylindrical opening 14. Cavity 18 is adapted toreceive optical and electronic processing equipment represented by block22, the workings of which will be described in detail later in thespecification. Optical fibers 24 and 26 extend from inside block 22,through cavity portion 20, and terminate in fixed positions in housing12 in optical communication with banded surface 28 on piston 16.Operating details of device 10 will be fully described after furtherintroductory coverage.

Housing 12 is adapted to be connected to some rigid or fixed structure(not illustrated), while the outer end of piston 16 terminating in head30 is adapted to be secured to body or object 32, such as a steel plate,interfacing with water 34. It is through this water that explosiveforces are transmitted against the plate. Explosive displacement ofplate 32 to the right, as viewed in FIG. 1, moves piston 16 to theright. This causes banded portion 28 of the piston to translate past thelight emitting ends of spaced apart optical fibers 24 and 26. Asillustrated in FIG. 2, banded end 28 of piston 16 is comprised of aplurality of annular lands 29 and grooves 29'. The purpose of thesebands is to present surfaces having alternating high and low lightreflectivity. One method of forming bands of different reflectivity isto machine spaced apart annular grooves 29' around piston 16 and leavethem separated by lands 29. The land surfaces may be polished for highlight reflectivity while the grooves are left as machined with little orno finishing for low light reflectivity. The bands may also be formed bynumerous types of surface treatment for obtaining differentreflectivity. Furthermore, the bands, regardless of form, need not beannular about the piston. They can be in side by side bar relationship.Regardless of form, the bands or bars are preferably of the same linearwidths, but this is not a critical requirement.

FIG. 3 is included in the drawing for illustrating a principle ofoptical coupling from one fiber to another. When portions of opticalfibers have their cladding removed and are fused together light willleak from one fiber (A) to another fiber (B), or vice versa. Thesefibers may be either twisted or otherwise retained in close adjacency.Another principle for coupling light between fibers is taught in U.S.Pat. No. 4,264,126, issued to Sang K. Sheem, where the cladding in thearea of light transfer is first etched away to allow light leakage. Thefibers are subsequently encased in a liquid or other substance having anappropriately lower index of refraction to prevent significant lightleakage.

A light emitting diode (LED) may be used as a light source (notillustrated) for introducing light into the end of optical fiber (A) atport 2. This light passes along the fiber, and, upon exiting from port1, strikes a reflector to be returned to Fiber (A) where it travels backin the reverse direction. Some of the light splits or divides into fiber(B) and exits port 4 into photodiode detector (not illustrated). Thisprinciple of light coupling is followed in FIG. 4. In FIG. 4, lightintroduced into the end of optical fiber 36 passes to optical coupler 38and exits the optical fiber at end 42. The light is reflected by thesurface of groove 29' of piston 16 back into the fiber where it travelsin the reverse direction to coupler 38. There the light divides, andpart of it is leaked into fiber 40 from which it exits to a lightdetector (not illustrated). The magnitude of light returned (reflected)into fiber 42 and coupled into optical fiber 40 depends upon thereflectivity of groove 29'. Upon linear movement of piston 16,alternating magnitudes of light reflected from surface 29' (lowreflectivity) and surface 29 (high reflectivity) produce a squaresignal, such as illustrated in FIG. 4. This is produced and fed to adetector. This signal can then be converted into an electrical signal bythe processes disclosed in FIGS. 6 and 7. One electric output signalalone, while representing displacement and rate of displacement ofpiston 16, would fail to indicate the direction of displacement. Byusing a dual optical arrangement as illustrated in FIG. 5, the directionof piston 16 can also be derived.

In the FIG. 5 embodiment, light is also launched into optical fiber 36'from another LED (not illustrated). This light passed through coupler38' and along the optical fiber to exit at end 26'. Upon striking thesurface of land 29, it reflects back into the fiber, through coupler38', where a portion is split into optical fiber 40' and fed to anotherphotodetector (not illustrated). The optical signal which exits opticalfiber 40' is processed in the same manner as described with reference toFIG. 4. The embodiment of FIG. 4 is included in FIG. 5. It will be notedthat the square signals emanating from optical fibers 40 and 40' areslightly out of phase. This allows the direction of travel of piston 16to be derived. The out of phase outputs are accomplished by placing thelight output ends of optical fibers 24 and 26' at non-half-integerspacings opposite bands 28 such that a phase shift occurs between thetwo fiber outputs. The distance X is not equal to 1/2 N D, where Dequals the ring separation and N equals integer. This provides theinformation needed to determine direction that piston 16 is traveling.When a translation from leading to lagging occurs between the two fiberoutputs, a change in direction of travel of piston 16 is indicated, asshown in FIG. 9.

Referring now to FIGS. 6 and 7 there is illustrated the electronics inbox 22 necessary for processing light signals emitted from opticalfibers 40 and 40'. A separate circuit is provided for each opticaloutput. Upon arriving at a detector, the optical signal is convertedinto an electrical signal by photodiode 50 and amplified. The photodiodeis operated in the short circuit mode. Following the photodiode sensoramplifier 50 is a gain amplifier 52 and a low pass filter 54 to removeany high frequency noise and signal variations. Following the filter isa comparator 56 which compares the signal level to a threshold level.When the threshold level is exceeded by detecting a high reflectivity,the output of the comparator will go high. For low reflectivity, a lowoutput of the comparator will occur. The detector signal is driventhrough a line driver 58 to an external recorder. Displacement of piston16 by plate 22 is calculated by the formula:

Displacement =ND where N is the number of pulses and D is the separationbetween rings (band 28 spacing). See FIG. 8 which is a representation ofthe output to the recorder from the fibers. Velocity is calculated bythe formula:

    Velocity=(1/T)/ (Dx(1/SC)×(PS/RS))

where

SC=stripchart speed

RS=record speed

PS=playback speed

T=threads per inch

Acceleration can be found by taking the derivative of the velocity.Thus, two channels of data are generated. By analysis of the twochannels, displacement, velocity and acceleration can be calculated.

Having thus disclosed an embodiment of the invention it will be apparentto ones skilled in the art how to practice the invention. It will alsobe apparent that changes and variations can be made thereto withoutdeparting from the spirit of the invention which is defined within thescope of the claims appended hereto.

What is claimed is:
 1. Apparatus for measuring direction, extent, andrate of linear displacement of a body subjected to shock forces,comprising:a housing adapted to be connected to a rigid structure;piston means received in the housing and adapted for linear displacementtherein; said piston means including a portion outside the housingconnectable with the body whereby shock forces on the body causes thepiston to be linearly displaced in the housing; said piston means havinga surface including a plurality of bands having alternating high and lowlight reflectivity; means carried by the housing for illuminatinglinearly spaced apart spots on the piston surface including the bands;means detecting information from band reflected light; and, meansconverting light information to electrical signals; whereby, when thepiston is displaced, the bands are caused to translate past theillumination for reflecting light information.
 2. The inventionaccording to claim 1 wherein the illuminating means includes a pair ofoptical fibers having ends terminating adjacent the bands for opticalcommunication therewith.
 3. The invention according to claim 2 whereinthe optical fiber ends are linearly spaced apart.
 4. The inventionaccording to claim 3 wherein the linear spacing of the optical fiberends is a distance which is not a multiple of the distance betweencorresponding locations on successive bands of like reflectivity.
 5. Theinvention according to claim 1 wherein the bands are annularly disposedabout the piston.
 6. The invention according to claim 1 wherein thebands are defined by a series of linearly spaced-apart annular groovesand lands therebetween.
 7. The invention according to claim 5 whereinthe bands are formed by spaced apart grooves formed in the pistonsurface.
 8. The invention according to claim 7 wherein the grooves havelower light reflectivity than lands between the grooves.
 9. Theinvention according to claim 1 wherein the optical fiber ends emit anilluminating spot on the piston surface of a size substantially smallerthan the width of any band.
 10. Apparatus for measuring direction,extent and rate of linear displacement of a body subjected to anunderwater shock by an explosion, comprising:a housing; piston meanscarried by the housing and having a portion extending therefrom; saidhousing adapted to be secured in a fixed position; said piston extensionconnectable to the body for linear displacement therewith whenever thebody is subjected to the explosive shock; said piston having a surfaceincluding a plurality of annular bands of alternating high and low lightreflectivity; light sources; a pair of optical fibers in communicationwith the light sources and terminating in linearly spaced apart ends forilluminating spots on the piston surface whereby emitted light isreflected by the bands back into respective optical fibers and carriedto respective detectors; and, means converting optical informationreceived by the detectors to electrical signals; whereby displacement ofthe piston is indicated by light reflecting from the bands as theytranslate past the optical fibers illuminating spots.
 11. The inventionaccording to claim 10 wherein the piston bands are annularly disposedabout the piston;
 12. The invention according to claim 11 wherein thebands are of equal linear width.
 13. The invention according to claim 12wherein the bands are formed by annular grooves with annular landtherebetween.
 14. The invention according to claim 13 wherein the landsare more optically reflective than the grooves.
 15. The inventionaccording to claim 12 wherein the optical fiber ends are linearly spacedapart a distance which is not a multiple of the distance betweencorresponding locations on successive bands of high reflectivity.